Power conversion systems of the DC link type convert AC power supplied by an AC power source into DC power on a DC link. One example of such a power system is a variable-speed, constant-frequency (VSCF) power generating system which converts AC power produced by a brushless, synchronous generator coupled to a variable-speed prime mover into DC power on a DC link which is in turn coupled to an inverter. The inverter is controlled to produce constant-frequency AC power from the DC power on the DC link which may in turn be filtered by a filter and provided to one or more AC loads.
In such types of systems, it is important that the harmonics in the power produced by the inverter be controlled so that the size and weight of the filter may be minimized. Typically, the switches of the inverter are controlled in accordance with switching patterns established under the assumption that the DC link is truly "stiff", i.e. the DC link conducts pure DC power with no substantial AC components. However, it has been found that AC components are introduced on the DC link due to operation of the rectifier and inverter as well as by vibration of loads connected to the inverter. These AC components in turn cause the output of the inverter to have increased harmonic content, thus requiring a relatively large and heavy filter. Moreover, the harmonic content in the inverter output varies during operation of the VSCF system in a manner which cannot be controlled by modification of the switching patterns for the inverter alone.